Cryptic diversity in the mtDNA of the ornamental fish Carnegiella strigata

Mitochondrial DNA ( mtDNA) sequences of the marbled hatchetfish Carnegiella strigata, an ornamental fish exported from the Negro River, was examined to determine its genetic diversity and population structure in blackwater rivers (Negro and Uatumã Rivers) in the central Amazon. Analyses of a 646 bp fragment of the ATPase 6/8 mtDNA gene revealed two monophyletic lineages of C. strigata with considerable genetic distance between them (10–12%), suggesting that these lineages should not be considered a single stock. Furthermore, there were strong differences in the geographical distribution of the lineages. These results indicate a past association between drainages of the Negro and Uatumã Rivers. They also suggest that, in the Negro River, its main tributary, the Branco River, may act as a geographical barrier and potentially an ecological barrier between populations of the middle and lower portions of the river.     

Cytogenetic variation of repetitive DNA elements in Hoplias malabaricus (Characiformes - Erythrinidae) from white,black and clear water rivers of the Amazon basin

Hoplias malabaricus is a common fish species occurring in white, black and clear water rivers of the Amazon basin. Its large distribution across distinct aquatic environments can pose stressful conditions for dispersal and creates possibilities for the emergence of local adaptive profiles. We investigated the chromosomal localization of repetitive DNA markers (constitutive heterochromatin, rDNA and the transposable element REX-3) in populations from the Amazonas river (white water), the Negro river (black water) and the Tapajós river (clear water), in order to address the variation/association of cytogenomic features and environmental conditions. We found a conserved karyotypic macrostructure with a diploid number of 40 chromosomes (20 metacentrics + 20 submetacentrics) in all the samples. Heteromorphism in pair 14 was detected as evidence for the initial differentiation of an XX/XY system. Minor differences detected in the amount of repetitive DNA markers are interpreted as possible signatures of local adaptations to distinct aquatic environments.     

The ecogeographical differentiation of Amazonian inundation forests

Due to the considerable annual fluctuations of water level of the Amazonian rivers, their river banks are fringed with periodically flooded forests of vast extension. The biota of these communities are adapted to annual inundations that can last for more than half a year. Water chemistry is most important for the floristic differentiation of these flooded forests. White water rivers, which carry a rich load of suspended material originating from the erosion of the Andes, have a floristic composition related to that of the noninundatable Amazonian forest. Clear water and black water rivers, which originate in the Amazon Basin or its adjacent crystalline shields, are nutrient-poor and more or less acidic; their flora is related to that of peculiar woodland and savannah vegetation on oligotrophic white sand. The distribution patterns of floodplain species of nutrient-poor waters point to a centre of diversity in the Upper Rio Negro region, and another one in the Guayana lowland. These coincide with diversity centres for species of non-flooded habitats. Hence it seems unlikely that species diversity is directly influenced by pluviosity. The flooded forests have developed biotic interactions with the fish fauna of the Amazon Basin, which are vital for their continued existence. It is assumed that the origin of these habitats, their biota and their interactions dates back long into the Tertiary.     

Wide but not impermeable: Testing the riverine barrier hypothesis for an Amazonian plant species

Wallace's riverine barrier hypothesis postulates that large rivers, such as the Amazon and its tributaries, reduce or prevent gene flow between populations on opposite banks, leading to allopatry and areas of species endemism occupying interfluvial regions. Several studies have shown that two major tributaries, Rio Branco and Rio Negro, are important barriers to gene flow for birds, amphibians and primates. No botanical studies have considered the potential role of the Rio Branco as a barrier, while a single botanical study has evaluated the Rio Negro as a barrier. We studied an Amazon shrub, Amphirrhox longifolia (A. St.‐Hil.) Spreng (Violaceae), as a model to test the riverine barrier hypothesis. Twenty‐six populations of A. longifolia were sampled on both banks of the Rio Branco and Rio Negro in the core Amazon Basin. Double‐digest RADseq was used to identify 8,010 unlinked SNP markers from the nuclear genome of 156 individuals. Data relating to population structure support the hypothesis that the Rio Negro acted as a significant genetic barrier for A. longifolia. On the other hand, no genetic differentiation was detected among populations spanning the narrower Rio Branco, which is a tributary of the Rio Negro. This study shows that the strength of riverine barriers for Amazon plants is dependent on the width of the river separating populations and species‐specific dispersal traits. Future studies of plants with contrasting life history traits will further improve our understanding of the landscape genetics and allopatric speciation history of Amazon plant diversity.     

Postglacial colonization of Northern Europe by distinct phylogeographic lineages of the bullhead, Cottus gobio

Three major phylogeographic lineages of the cottid fish Cottus gobio (bullhead) were identified in northern Europe from mitochondrial DNA sequences and allozyme data. The largely separate freshwater distributions of the lineages demonstrate distinct postglacial colonization histories. West of the Baltic Sea, Swedish lakes were invaded from the southwest (Germany). Another, eastern lineage has colonized the inland waters northeast and east of the Baltic, from refugia in northwest Russia; this lineage comprises a distinct subgroup found only from Estonia. The third lineage, found south and southeast of the Baltic, probably descended from rivers draining to the Black Sea from the north (e.g. Dnepr). In coastal waters of the Baltic Sea, and in near-coast inland waters, the lineages are now found intermixed in various combinations. The alternating fresh- and saltwater phases of the Baltic basin have variously enabled and disabled the use of coastal waters as colonization routes. Hypotheses on the chronology of dispersal and lineage mixing can be based on the distribution of the marker genes and the paleohydrographical record. The diversity of the Fennoscandian bullhead thus comprises anciently diverged (probably mid-Pleistocene) refugial lineages that in their freshwater range constitute distinct evolutionarily significant units. The thorough mixing of the various genomic origins in and around the Baltic, however, refutes the controversial view of distinct species status for the western and eastern ('Cottus koshewnikowi') bullheads. The postglacial contact of the lineages has created new diversity that cannot be interpreted in a conventional hierarchical framework of taxonomic or conservation units.     

Cryptic diversity in the lizard genus Plica (Squamata): phylogenetic diversity and Amazonian biogeography

Intra‐ and interspecific genetic diversity of the lizard species Plica plica (9 localities) and Plica umbra (19 localities) from the Brazilian Amazon was analysed using two mitochondrial (16S rDNA and CO1) and one nuclear (prolactin receptor – PRLR) genes. We generated a maximum‐likelihood and Bayesian hypotheses of phylogenetic relationships, and using the bPTP and ABGD lineage delimiting methods inferred the most likely number of lineages within each species. Both methods delimited five distinct lineages in Plica plica and six lineages within Plica umbra. The nominal subspecies of Plica umbra was comprised of one lineage, while Plica umbra ochrocollaris was comprised of five lineages. In majority of the cases, lineages were restricted to the interfluves of major Amazonian rivers, and different lineages occupied distinct areas of endemism. Phylogenetic relationships of the lineages are largely concordant with the hypothesized formation of the areas of endemism. The geographic structuring of the clades and the delimitation of these clades as distinct lineages suggest the possibility that these lineages represent species. If the observed diversity of lineages within the genus Plica is characteristic of squamate reptiles of the Amazon region, the diversity of squamates is grossly underestimated.     

A marine fish follows Wallace's Line: the phylogeography of the three-spot seahorse (Hippocampus trimaculatus, Syngnathidae, Teleostei) in Southeast Asia

Aim To test the potential of two contrasting biogeographical hypotheses (‘Indian/Pacific Ocean Basin’ vs. ‘Wallace's Line’) to explain the distribution of genetic diversity among populations of a marine fish in Southeast Asia. Location The marine waters of Asia and Southeast Asia: from India to Japan, and east to the Indonesian islands of Sulawesi and Flores. Methods We sequenced a 696 base pair fragment of cytochrome b DNA of 100 individuals of Hippocampus trimaculatus Leach 1814 (three‐spot seahorse), obtained from across its range. We tested our hypotheses using phylogenetic reconstructions and analyses of molecular variance. Results Significant genetic divergence was observed among the specimens. Two distinct lineages emerged that diverged by an average of 2.9%. The genetic split was geographically associated, but surprisingly it indicated a major east–west division similar to the terrestrial Wallace's Line (Φ_ST = 0.662, P < 0.001) rather than one consistent with an Indian‐Pacific ocean basin separation hypothesis (Φ_ST = 0.023, P = 0.153). Samples from east of Wallace's Line, when analysed separately, however, were consistent with an Indian/Pacific Ocean separation (Φ_ST = 0.461, P = 0.005). The degree of genetic and geographical structure within each lineage also varied. Lineage A, to the west, was evolutionarily shallow (star‐like), and the haplotypes it contained often occurred over a wide area. Lineage B to the east had greater genetic structure, and there was also some evidence of geographical localization of sublineages within B. Main conclusions Our results indicate that the genetic diversity of marine organisms in Southeast Asia may reflect a more complex history than the simple division between two major ocean basins that has been proposed by previous authors. In particular, the east–west genetic division observed here is novel among marine organisms examined to date. The high haplotype, but low nucleotide diversity to the west of Wallace's Line is consistent with post‐glacial colonization of the Sunda Shelf. Additional data are needed to test the generality of these patterns.     

Sexual selection: Another Darwinian process

Why was sexual selection so important to Darwin? And why was it de-emphasized by almost all of Darwin's followers until the second half of the 20th century? These two questions shed light on the complexity of the scientific tradition named “Darwinism”. Darwin's interest in sexual selection was almost as old as his discovery of the principle of natural selection. From the beginning, sexual selection was just another “natural means of selection”, although different from standard “natural selection” in its mechanism. But it took Darwin 30 years to fully develop his theory, from the early notebooks to the 1871 book The Descent of Man, and Selection in Relation to Sex. Although there is a remarkable continuity in his basic ideas about sexual selection, he emphasized increasingly the idea that sexual selection could oppose the action of natural selection and be non adaptive. In time, he also gave more weight to mate choice (especially female choice), giving explicit arguments in favor of psychological notions such as “choice” and “aesthetic sense”. But he also argued that there was no strict demarcation line between natural and sexual selection, a major difficulty of the theory from the beginning. Female choice was the main reason why Alfred Russel Wallace, the co-discoverer of the principle of natural selection, engaged in a major controversy with Darwin about sexual selection. Wallace was suspicious about sexual selection in general, trying to minimize it by all sorts of arguments. And he denied entirely the existence of female choice, because he thought that it was both unnecessary and an anthropomorphic notion. This had something to do with his spiritualist convictions, but also with his conception of natural selection as a sufficient principle for the evolutionary explanation of all biological phenomena (except for the origin of mind). This is why Wallace proposed to redefine Darwinism in a way that excluded Darwin's principle of sexual selection. The main result of the Darwin–Wallace controversy was that most Darwinian biologists avoided the subject of sexual selection until at least the 1950 s, Ronald Fisher being a major exception. This controversy still deserves attention from modern evolutionary biologists, because the modern approach inherits from both Darwin and Wallace. The modern approach tends to present sexual selection as a special aspect of the theory of natural selection, although it also recognizes the big difficulties resulting from the inevitable interaction between these two natural processes of selection. And contra Wallace, it considers mate choice as a major process that deserves a proper evolutionary treatment. The paper's conclusion explains why sexual selection can be taken as a test case for a proper assessment of “Darwinism” as a scientific tradition. Darwin's and Wallace's attitudes towards sexual selection reveal two different interpretations of the principle of natural selection: Wallace's had an environmentalist conception of natural selection, whereas Darwin was primarily sensitive to the element of competition involved in the intimate mechanism of any natural process of selection. Sexual selection, which can lack adaptive significance, reveals this exemplarily.     

A captive breeding experiment reveals no evidence of reproductive isolation among lineages of a polytypic poison frog

Reproductive isolation is central to the generation of biodiversity, yet a clear understanding of the contributions of alternative reproductive barriers to this process remains elusive. Studies of young lineages that have diverged in ecologically important traits can offer insights into the chronology and relative importance of various isolating mechanisms during speciation. In poison frogs (Dendrobatidae), within‐species lineages often differ dramatically in coloration, a trait subject to natural and sexual selection. Coloration in the strawberry poison frog (Oophaga pumilio) is particularly diverse and previous work suggests the potential for reproductive isolation. We used a captive breeding experiment to assess the extent of reproductive isolation among three allopatric, genetically distinct O. pumilio lineages that differ in coloration. We compared reproduction of within‐ and between‐lineage pairs, predicting that if lineages are isolated, within‐lineage pairs would be most successful. We also examined the fertility and productivity of F1 backcrosses of admixed offspring. We found no evidence suggesting behavioural pre‐zygotic or post‐zygotic reproductive isolation, indicating that isolation would not be maintained by intrinsic mechanisms in the event of secondary contact. Future work should address costs of between‐lineage matings exerted by extrinsic natural and/or sexual selection against admixed offspring.     

Effects of salinity on distribution, growth and survival of three neotropical armoured catfishes (Siluriformes—Callichthyidae)

Three species of closely related armoured catfishes, Hoplosternum littorale, H. thoracatum and Callichthys callichtkys , are widely distributed throughout (sub)tropical South America, but a detailed inspection of collection localities in both Suriname and the South American continent reveals a clear pattern. Hoplosternum littorale is not found in rainforest creeks and clear-water rivers draining the Precambrium Guiana and Brazilian Shields. Here, waters are extremely poor in dissolved minerals. In salinity tolerance experiments, 1-week-old larvae of H. littorale were found to be incapable of surviving in rainwater that is also very poor in electrolytes. Probably for the same reason, H. littorale is not collected in black-water rivers like the Rio Negro. Within the Amazon river-system H. litiorale is found in sediment-laden white-water rivers originating in the Andes. Larvae of H. thoracatum and C. caiiichthys are able to survive and develop further in rainwater and consequently have a less restricted distribution than H. littorale . Both species are found in clear water and black water. All three species tolerate salinities up to 2000 mg Cl/1, and show a marked decrease in growth at low salinities.     

The contribution of long‐term isolated water bodies to floodplain fish diversity

1. Floodplains and their water bodies are typical ecotopes of large lowland rivers. The lowland Oder River, Germany, provided a rare opportunity to study fish assemblages of comparable floodplain water bodies differing by >50 years of isolation history. We hypothesised that true floodplain specialists peak in rarely connected water bodies, while frequently flooded waters support tolerant generalists. 2. Three macrohabitats, main channel (MC), temporarily connected and isolated floodplain water bodies, were sampled by electro‐fishing and their fish assemblage characters recorded. 3. Long‐term isolation of floodplain water bodies had a significant effect on the fish assemblage by promoting species preferring still water. Limnophilic and floodplain specialist species significantly increased with isolation. 4. Fish densities, species richness and diversity clearly differed between MC sites and floodplain water bodies. Shannon’s species diversity index peaked in both the MC and isolated waters and was lowest in the temporarily flooded waters where eurytopic fish dominated. 5. The significant gain in abundance and numbers of limnophilic species in the isolated compared to the temporarily flooded water bodies underpinned the significant contribution of long‐term isolated waters to the gamma‐diversity of large floodplain rivers, which should be considered in floodplain rehabilitation.     

Tangled banks: A landscape genomic evaluation of Wallace's Riverine barrier hypothesis for three Amazon plant species

Wallace's Riverine Barrier hypothesis is one of the earliest biogeographic explanations for Amazon speciation, but it has rarely been tested in plants. In this study, we used three woody Amazonian plant species to evaluate Wallace's Hypothesis using tools of landscape genomics. We generated unlinked single‐nucleotide polymorphism (SNP) data from the nuclear genomes of 234 individuals (78 for each plant species) across 13 sampling sites along the Rio Branco, Brazil, for Amphirrhox longifolia (8,075 SNPs), Psychotria lupulina (9,501 SNPs) and Passiflora spinosa (14,536 SNPs). Although significantly different migration rates were estimated between species, the population structure data do not support the hypothesis that the Rio Branco—an allopatric barrier for primates and birds—is a significant genetic barrier for Amphirrhox longifolia, Passiflora spinosa or Psychotria lupulina. Overall, we demonstrated that medium‐sized rivers in the Amazon Basin, such as the Rio Branco, are permeable barriers to gene flow for animal‐dispersed and animal‐pollinated plant species.     

Pleistocene sea level fluctuations and the phylogeography of the dugong in Australian waters

We investigated phylogeography, demography, and population connectivity of the dugong (Dugong dugon) in Australian waters using mitochondrial control region DNA sequences from 177 Australian dugongs and 11 from elsewhere. The dugong is widespread in shallow Indo‐West Pacific waters suitable for growth of its main food, seagrass. We hypothesized that the loss of habitat and creation of a land barrier (the Torres Strait landbridge) during low sea level stands associated with Pleistocene glacial cycles have left a persisting genetic signature in the dugong. The landbridge was most recently flooded about 7,000 yr ago. Individual dugongs are capable of traveling long distances, suggesting an alternative hypothesis that there might now be little genetic differentiation across the dugong's Australian range. We demonstrated that Australian dugongs fall into two distinct maternal lineages and exhibit a phylogeographic pattern reflecting Pleistocene sea‐level fluctuations. Within each lineage, genetic structure exists, albeit at large spatial scales. We suggest that these lineages diverged following the last emergence of the Torres Strait landbridge (ca. 115 kya) and remained geographically separated until after 7 kya when passage through Torres Strait again became possible for marine animals. Evidence for population growth in the widespread lineage, especially after the last glacial maximum, was detected.     

Nest shape explains variation in sexual dichromatism in New World blackbirds

Following Charles Darwin, research on sexual dichromatism has long focused on sexual selection driving ornamentation in males. However, Alfred Russel Wallace proposed another explanation – that dichromatism evolves as a result of selection favoring crypsis in incubating females. Many recent studies suggest that evolutionary changes in sexual dichromatism often result from changes in female, in addition to male, plumage, yet the evolutionary mechanisms driving changes in female plumage remain largely unexplained. To test Wallace's hypothesis, we examined variation in sexual dichromatism and nest shape, a proxy for predation risk, among New World blackbirds (Aves: Icteridae). Phylogenetic models reveal an evolutionary correlation between sexual dichromatism and nest exposure. Specifically, we found that transitions in monochromatic lineages with exposed nests toward either concealed nests or dichromatism were common. Although this evidence supports Wallace's hypothesis that female incubation leads to selection for crypsis or concealment, we also found that transitions to monomorphism were common, even in lineages with exposed nests – a result suggestive of a role for positive selection on female ornamentation. These patterns of plumage evolution support a growing body of work emphasizing the importance of developing and testing hypotheses to explain evolutionary changes in female, as well as male, ornamentation.     

PARALLEL EVOLUTION OF LAKE-STREAM PAIRS OF THREESPINE STICKLEBACKS (GASTEROSTEUS) INFERRED FROM MITOCHONDRIAL DNA VARIATION

Three drainage systems in British Columbia, Canada, contain divergent parapatric lake-stream pairs of threespine sticklebacks (Gasterosteus aculeatus): Drizzle and Mayer Lakes on Graham Island, Queen Charlotte Islands, and Misty Lake on northeastern Vancouver Island. Ecological and morphological differences between members of all three lake-stream pairs are strikingly similar; lake fish are melanistic and slim bodied with smaller mouths and more gill rakers than the mottled-brown and robust-bodied stream sticklebacks. We estimated the level of genetic divergence between lake and stream fish in Misty Lake and tested hypotheses of single versus multiple origins of the pairs by assaying mitochondrial DNA (mtDNA) restriction site variation in samples from the three lake systems. MtDNA analysis revealed the existence of two highly divergent lineages differing by 2.7% in sequence. One lineage predominated in Misty stream fish (73%), whereas the other lineage predominated in Misty Lake samples (96%). Comparable forms (lake or stream) in the different lakes did not cluster together in terms of mtDNA nucleotide divergence, suggesting that the pairs have had independent origins. We concluded that: (1) divergent mtDNA lineages in North Pacific sticklebacks stem from historical isolation in the two major glacial refugia proposed for the North Pacific (Beringia and Cascadia); (2) the stream and lake pair in Misty Lake are distinct gene pools; (3) the divergence between parapatric lake and stream Gasterosteus represents parallel evolution having occurred at least twice in the North Pacific; and (4) different scales of evolutionary divergence exist in North Pacific Gasterosteus, that is, a relatively ancient divergence of mtDNA clades as well as recent (i.e., postglacial) divergence of ecotypes within major clades.     

Effects of DOM photochemistry on bacterial metabolism and CO2 evasion during falling water in a humic and a whitewater river in the Brazilian Amazon

In the Amazon river system, the source of the large quantity of CO₂ evading from river surfaces remains unidentified. Photochemical transformation of dissolved organic matter (DOM) into dissolved inorganic carbon (DIC) and low molecular weight organic acids (LMWOAs) is a promising candidate. Few studies in the Amazon river system, and river systems in general, have attempted to quantify the contribution of these specific photoproducts to CO₂ evasion. We conducted photochemical degradation and ¹⁴C addition experiments to measure the rate of production and the impact on bacterial metabolism, respectively, in the black water Rio Negro and in the white water Rio Solimões during low water. We found statistically significant production of both photoproducts in the Rio Negro and none in Rio Solimões. We also found that two photochemically produced LMWOAs??acetic and formic acid??may play a significant role in bacterial metabolism in both rivers. Based on our experimental results, we estimate that photochemically produced CO₂, acetic acid and formic acid alone contribute to only 0.5% of the CO₂ evading from the Rio Negro. Due to our experimental set-up, analytical methods and time of sampling, we caution that our estimate is very conservative. More extensive research is needed before drawing conclusions on the contribution of photochemistry to CO₂ evasion from river surfaces of the Amazon basin.     

Perspectives Poulton,Wallace and Jordan: How discoveries in Papilio butterflies led to a new species concept 100 years ago

A hundred years ago, in January 1904, E.B. Poulton gave an address entitled ‘What is a species?’ The resulting article, published in the Proceedings of the Entomological Society of London, is perhaps the first paper ever devoted entirely to a discussion of species concepts, and the first to elaborate what became known as the ‘biological species concept’. Poulton argued that species were syngamic (i.e. formed reproductive communities), the individual members of which were united by synepigony (common descent). Poulton's species concept was informed by his knowledge of polymorphic mimicry in Papilio butterflies: male and female forms were members of the same species, in spite of being quite distinct morphologically, because they belonged to syngamic communities. It is almost certainly not a coincidence that Alfred Russel Wallace had just given Poulton a book on mimicry in December 1903. This volume contained key reprints from the 1860s including the first mimicry papers, by Henry Walter Bates, Wallace himself and Roland Trimen. All these papers deal with species concepts and speciation as well as mimicry, and the last two contain the initial discoveries about mimetic polymorphism in Papilio: strongly divergent female morphs must belong to the same species as non‐mimetic males, because they can be observed in copula in nature. Poulton, together with his contemporaries Karl Jordan and Walter Rothschild, who had monographed world Papilionidae, were strongly influential on the evolutionary synthesis 40 years later. Ernst Mayr, in particular, had collected birds and butterflies for Walter Rothschild, and had visited Tring, where Jordan worked, in the 1920s. The recognition of different kinds of reproductive and geographic isolation, the classification of isolating mechanisms, the use of the term sympatry, and the biological species concept all trace back to Poulton's 1904 paper. Poulton's paper, in turn, inherits much from Wallace's 1865 paper on Asian Papilio contained in the very book Wallace gave Poulton a month earlier. Wallace's gift, and Poulton's subsequent New Year address are thus key events in the history of species concepts, systematics and evolutionary biology.     

Crossing the (Wallace) line: local abundance and distribution of mammals across biogeographic barriers

Past and ongoing vertebrate introductions threaten to rearrange ecological communities in the Indo‐Malay Archipelago, one of Earth's most biodiverse regions. But the consequences of these translocations are difficult to predict. We compared local abundance and distributions in four tropical mammal lineages that have crossed from Asia to Wallacea or New Guinea. The local abundance of macaques (Macaca spp.), which naturally crossed Wallace's Line, was higher in Sulawesi (east of the line; mean = 3.7 individuals per camera station, 95% CI = 2.2: 5.1) than in Borneo (west of the line; mean = 1.1, CI = 0.8: 1.4), but the local abundance of Malay civets (Viverra tangalunga), Rusa deer, and Sus pigs was similar in their native ranges and where they had been introduced by humans east of Wallace's Line. Proximity to rivers increased Malay Civet local abundance and decreased the local abundance of pigs in parts of their introduced ranges (Maluku and New Guinea, respectively), while having no effect on local abundance in their native ranges (Borneo) or other areas where they have been introduced (Sulawesi). That local abundance was higher east of Wallace's Line in just one of four mammal lineages is consistent with findings from plant invasions, where most species have similar abundance in their native and introduced ranges. However, species’ ecology may change as they enter new communities, for example, their patterns of abundance at local scales. This could make it difficult to predict community structure in the face of ongoing species introductions.     

Methane emissions from Amazonian Rivers and their contribution to the global methane budget

Methane (CH₄) fluxes from world rivers are still poorly constrained, with measurements restricted mainly to temperate climates. Additional river flux measurements, including spatio‐temporal studies, are important to refine extrapolations. Here we assess the spatio‐temporal variability of CH₄ fluxes from the Amazon and its main tributaries, the Negro, Solimões, Madeira, Tapajós, Xingu, and Pará Rivers, based on direct measurements using floating chambers. Sixteen of 34 sites were measured during low and high water seasons. Significant differences were observed within sites in the same river and among different rivers, types of rivers, and seasons. Ebullition contributed to more than 50% of total emissions for some rivers. Considering only river channels, our data indicate that large rivers in the Amazon Basin release between 0.40 and 0.58 Tg CH₄ yr^?1. Thus, our estimates of CH₄ flux from all tropical rivers and rivers globally were, respectively, 19–51% to 31–84% higher than previous estimates, with large rivers of the Amazon accounting for 22–28% of global river CH₄ emissions.     

How Charles Darwin received Wallace's Ternate paper 15 days earlier than he claimed: a comment on van Wyhe and Rookmaaker (2012)

Van Wyhe and Rookmaaker (2012) postulate a set of events to support their claim that Wallace's ‘evolution’ letter, posted at Ternate in the Moluccas in the spring of 1858, arrived at Darwin's home on 18 June 1858. If their claim were to be proven, then evidence that Darwin probably received Wallace's letter 2 weeks earlier than he ever admitted would clearly be erroneous, and any charges that he plagiarized the ideas of Wallace from that letter would be shown to be wrong. Here, evidence against this interpretation is presented and it is argued that the letter did indeed arrive in the port of Southampton on 2 June 1858 and would have been at Darwin's home near London the following day. If this were true, then the 66 new pages of material on aspects of Divergence that Darwin entered into his ‘big’ species book in the weeks before admitting he had received the letter could be interpreted as an attempt to present Wallace's ideas as his own. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 472–477.